The present invention relates to an intravascular catheter. In particular, the present invention relates to an intravascular catheter for use in percutaneous transluminal angioplasty (PTA) in large peripheral vessels, for example the legs and kidneys, that have an inside diameter in the range of 4-8 mm.
Angioplasty procedures have gained wide acceptance in recent years as an efficient and effective method for treating various types of vascular disease, particularly percutaneous transluminal coronary angioplasty (PTCA). The most widely used form of angioplasty makes use of a dilatation catheter which has an inflatable balloon at its distal end. The catheter is percutaneously inserted into the patient's vascular system and is maneuvered from its proximal end through the vascular system. The physician uses a fluoroscope to guide the dilatation catheter through the vascular system until the un-inflated balloon is positioned across the stenosis. The catheter should be sufficiently flexible to flex around the acute bends that it encounters along the vascular system. The balloon is then inflated by supplying fluid under pressure, through an inflation-deflation lumen, to the balloon. Inflation of the balloon causes the vessel lumen to increase in diameter and thus reestablish acceptable blood flow through the vessel.
It can become necessary, during an angioplasty procedure, to quickly deflate and or remove the balloon catheter. For this reason it is important that the inflation-deflation lumen of the catheter be of sufficient size to permit rapid deflation of the balloon. In addition to the major factors, size and geometry of the inflation-deflation lumen, the walls of the inflation-deflation lumen should have a low resistance to fluid flow.
Catheters for use in PTA and PTCA have undergone a continuous evolution over the past 20 years particularly insofar as certain physical characteristics. There has been a continuing effort to reduce the profile or shaft size of dilatation catheters, so that the catheter can not only reach but also extend across a very tight stenosis. The profile or shaft size as well as the wall thickness of both the inner and outer shaft have been reduced in an effort to minimize the profile.
One of the most widely used angioplasty catheters is referred to as a "coaxial-over-the-wire" catheter. A coaxial-over-the-wire catheter is one in which a separate guide wire lumen is provided in the catheter so that a guide wire can be used to establish the path through the vascular system. The dilatation catheter can then be advanced over the guide wire until the balloon carried by the catheter is positioned across the stenosis. The guide wire lumen in a conventional coaxial-over-the-wire catheter is defined by an inner shaft that extends beyond the distal end of the catheter outer shaft. The proximal end of the balloon is secured to the outer distal end of the outer shaft. The distal end of the balloon is secured to the outer surface of the inner shaft. As a result of this conventional construction, both the inner and outer shafts are coextensive for all but the balloon area of the catheter. Both the inner and outer shafts of an over-the-wire catheter contribute to its "trackability" and "pushability." However, the outer shaft terminates short of the balloon area of the catheter and in this area the outer shaft does not function to support the thin walled inner shaft. In "coaxial-over-the-wire" catheters, the inside diameter of the inner shaft should be slightly larger than the outer diameter of the guide wire to permit the catheter to freely slide along the guide wire. Thus the outer diameter of the guide wire establishes the inner diameter of the inner shaft. In current catheters the inner shaft typically has a very thin wall thickness. Thus for a catheter having a given profile, the very thin inner shaft wall thickness establishes a maximum for the cross sectional area of the annulus shaped lumen between the inner and outer shaft. In catheters of this type the annulus shaped lumen, defined by the outer surface of the inner shaft and the inner surface of the outer shaft, functions as the inflation-deflation lumen. The cross sectional area of this annulus shaped lumen is the major factor in establishing the deflation time of the balloon. It is apparent that to increase the cross-sectional area of this lumen, in a catheter having a fixed profile or outer diameter, the wall thickness of the inner and or outer shafts must be decreased.
Dilation catheters must be sufficiently flexible to pass through tight curves or radii in the vascular system. The ability of a catheter to bend and advance through the vascular system is commonly referred to as trackability of the catheter. Thin shaft sections improve the trackability of a balloon catheter, however, if the walls become too thin, the tube sections tend to kink, collapse or burst. The resistance to kinking, collapsing or bursting can be increased by incorporating into a catheter shaft a coil spring in which the turns are spaced from each other so as to not compromise flexibility of the catheter. The inner shaft should have the ability to withstand being crushed by the pressure of the inflation fluid when the balloon is inflated and other mechanical pressures that it is exposed to, such as the uninflated balloon being wrapped around it for insertion purposes. The inner shaft is exposed to some of these mechanical forces at a time when the guide wire is not extending through the inner lumen and thus must rely on its own structure to resist such outside forces. The inner lumen must be maintained when the guide wire is not extending through it to enable the physician to perform distal dye injections and distal pressure measurements."
As catheters are being advanced through the vascular system they must be flexed to follow the sharp bends that are encountered in the vascular system. The un-inflated balloon is wrapped around the inner shaft, such that there are several layers of balloon material wrapped around the inner shaft during the catheter insertion procedure. The balloon material is stiff and thus has a tendency to kink or buckle. When the balloon area of the catheter is being advanced through a sharp bend, there is a risk that the inner surface of the un-inflated and folded balloon will engage the outer surface of the thin walled inner shaft. Such engagement could kink or collapse the thin walled inner shaft causing it to engage the guide wire within the inner shaft and resist or prevent advancement of the catheter along the guide wire. The possibility of the thin inner shaft kinking in the balloon area increases with larger balloon diameters. Although the wall thickness of the balloon is very thin, as the balloon diameters increase, the total amount of material contained in the balloon increases dramatically.
U.S. Pat. No. 4,994,032 discloses a vascular catheter including a "coaxial-over-the-wire" embodiment in which the inner and outer shafts are formed of flexible material including polyolefins, such as polyethylene, polypropylene, ethylene-propylene copolymers or ethylene-vinyl acetate copolymers, thermoplastic resins, such as polyvinyl chloride, polyamide elastomers or polyurethane, silicone rubber or latex rubber. The distal end of the inner shaft extends beyond the distal end of the outer shaft and a balloon is secured to the distal end portions of both the inner and the outer shafts. A reinforcement is wound about a portion of the outer surface of the inner shaft in the area enclosed within the balloon to thus render this section of the inner shaft more resistance against buckling and or breaking. The reinforcement is made from a coiled wire formed of an X-ray opaque material to aid in obtaining a clear fluoroscopy image. The catheter disclosed in this patent provides for a reinforcement member in the vulnerable balloon area of the catheter, however the reinforcement disclosed in this patent does not protect the inner shaft in the area immediately below the distal end of the outer shaft. This unprotected area immediately below the distal end of the outer shaft is the most vulnerable area of the inner shaft and becomes even more vulnerable when the reinforcement disclosed in the '032 patent is utilized. When the catheter is bent while negotiating an acute bend in the blood vessel, the inner shaft engages the inner corner of the distal edge of the outer shaft. When this occurs the inner corner of the distal edge acts as a fulcrum about which the inner shaft bends. With all the pressure concentrated at this location the inner shaft is very vulnerable to collapsing or breaking at this point.
It is not sufficient to provide reinforcement only to the most vulnerable areas of the catheter if other areas of the catheter are also vulnerable. The catheter should have acceptable trackability and should be supported along its entire length such that it has the ability to be bent around tight radii without collapsing or kinking. In some catheters such protection against collapsing can be accomplished by integrating a reinforcing coil, that is constructed such that its turns are not in contact with each other, along the entire length of the inner shaft and in other catheters such a coil with spaces between the turns need only extend across the balloon area. Also the reinforcing coil need not be continuous and can function satisfactorily if separated into separated sections. This should be accomplished without violating the profile requirements and the internal lumen size requirements. Thus there is a need for a catheter of a given profile that has equivalent mechanical integrity as current catheters of the same profile but which is constructed of inner and outer shafts that have a combined wall thickness that is less than that of current catheters.